Business machines, home applicances, and other types of equipment use solenoids to convert electrical power into mechanical motion. Generally, they use alternating current (A.C.) solenoids because of the availability of A.C. power from wall outlets. The primary fault with A.C. solenoids is the buzzing when the plunger is retracted into its home position. Diswashers, washing machines and office equipment are characterized by noisy solenoids, which is a distraction for prople that have to work around them.
Direct current (D.C.) solenoids, on the other hand, do not exhibit this buzzing when their plungers are retracted into their home positions. It is known to those skilled in the art, that inch per inhc an A.C. solenoid has more pull-in force when the plunger is extended than a D.C. solenoid and to obtain equivalent pull-in force, the D.C. solenoid is going to be considerably larger than the A.C. solenoid, and will also require a more powerful D.C. power supply. Conversely, it is known that inch per inch a D.C. solenoid has a higher hold-in force than an A.C. solenoid of equivalent structure.
In most business machines and primarily electrophotographic copy machines that incorporate a radiant energy type fixing device, a fail-safe safety system is used to prevent fires or some other type of event that could be dangerous to the personnel in the area and or the machine. These safety systems have some mechanism that is normally biased to a safety position so that if power is interrupted the mechanism will always move to this position to prevent a particular catastrophic event. A solenoid is usually employed to move the mechanism from this safety position to an operative position so that the machine can perform its function. In the case of copy machines using radiant energy type fixing devices, the solenoid is energy to hold the safety mechanism in its operative position throughout the entire working day so that the copy machine is always in a mode to make copies without a long warm up time. Generally A.C. solenoids are used in this type of application because of their high pull-in force needed to overcome the biasing means that pulls the mechanism into its safety position and because A.C. solenoids will dissipate less power in the home position than comparable D.C. solenoids. The buzzing characteristic of A.C. solenoids is very distracting for people that have to work around the equipment, especially in an office environment where a noisy copier can lower the efficiency of the office staff. Therefore it would be very desirable in a fail-safe safety system, to have a solenoid and or its driving circuit that would have the pull-in force characteristic of an A.C. solenoid, the hold-in force and quietness of a D.C. solenoid with the lowest power dissipation possible when the solenoid plunger is in its home position.